1. Field of the Invention
The present invention relates to acceleration sensor assemblies used in motor vehicles, and more particularly to acceleration sensor assemblies of a type which controls air bags, electronically controlled suspensions and the like.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional acceleration sensor assembly of the above-mentioned type will be described with reference to FIG. 6 of the accompanying drawings.
In the drawing, denoted by numeral 1 is a base board of stainless steel. A ceramic plate 3 on which predetermined circuits 2 are printed is tightly disposed through a suitable adhesive. On the ceramic plate 3, there are mounted an operational amplifier (Silicon-chip) 4 and resistors 5. Designated by numeral 6 is a piezoelectric type cantilever sensor proper which is disposed on the base board 1. The sensor proper 6 comprises a cantilever 61 which has a piezoelectric ceramic plate (not shown) connected thereto and a hermetically sealed casing 62 which contains the cantilever 61. Within the casing 62, there is filled a damping material such as a silicon gel. Designated by numerals 7 are terminal eyes which are stationarily held in openings 8 formed in the base board 1. For the holding of the terminal eyes 7, a suitable hermetically sealing agent 9 is applied to each opening 8.
Terminals of the operational amplifier 4 and the circuits 2 are connected through bonding wires 10 by means of a known wire bonding technique. Terminals of the sensor proper 6 and the circuits 2 are connected through bonding wires 11 by means of the wire bonding technique. Terminals 12 arranged on the ceramic plate 3 and the terminal eyes 7 are connected through bonding wires 13 by means of the same bonding technique.
However, due to its inherent construction, the above-mentioned conventional sensor assembly has the following drawbacks.
Since the terminals of the operational amplifier 4 are small in size and densely arranged, usage of thicker bonding wires as the wires 10 is substantially impossible. That is, the connection between the circuits 2 and the operational amplifier 4 has to be made by using relatively thinner bonding wires 10. Furthermore, because the sensor proper 6 is positioned at a distance from the circuits 2 on the ceramic plate 3, it is necessary to use relatively long bonding wires as the wires 11. This induces the need of using durable and thus relatively thicker bonding wires as the wires 11. Furthermore, since the bonding wires 13 between the terminals 12 of the ceramic plate 3 and the terminal eyes 7 have to bear against a relatively high current, the wires 13 must have each a relatively large cross section.
As is understood from the above, in this conventional acceleration sensor assembly, it is necessary to use bonding wires of different sizes as the wires 10, 11 and 12. However, bonding of such different-sized bonding wires to the desired terminal parts tends to make the bonding operation very troublesome and time-consuming. That is, the bonding of different-sized wires must be made by using several bonding machines, each machine being set to handle only one type of the bonding wires 10, 11 or 12.
Furthermore, the step for filling the casing 62 with the damping material lowers the productivity of the sensor proper 6 and thus causes increase in production cost. Furthermore, because of presence of the damping material, the size of the sensor proper 6 becomes increased and thus the entire of the sensor assembly becomes bulky in construction.